Alkyl silicate coatings are used mainly as anti-corrosive primers on steel substrates due to outstanding corrosion resistance, superior mechanical properties, very high temperature resistance and excellent chemical resistance.
Alkyl silicate coating compositions are typically one or two-component products. If the product is two-component, component one normally consists of a liquid mixture of the silicate binder, co-binders, anti-sagging agents and solvents and component two contains a metallic pigment, most often a metallic zinc pigment commonly referred to as zinc dust.
Zinc silicates are superior to other linings concerning anticorrosive properties due to its function as a sacrificial anode applied directly on top of the clean steel surface. When the coating layer is damaged the zinc will protect the damage by galvanic protection. The function is similar to that of galvanized steel.
Alkyl silicate binders are normally used as primer, i.e. first coating layer in a multiple coat system with subsequent layers of suitable generic types of coatings e.g. epoxy or polyurethane. However, silicate binders can also be used as one-coat systems or alternatively as topcoats
Alkyl silicates such as ethyl silicates are not reactive enough to be suitable for binder use without further hydrolysis. The hydrolysis of ethyl silicate can be either acid or base catalyzed. Acid catalysis generally proceeds in a somewhat slower and more controlled fashion. In addition, the presence of acids tends to stabilize reactive silanol (Si—OH) groups and increase storage stability. An overview of the issues of stability and reactivity for ethyl silicates is given in “The Use of Ethyl Silicate in Zinc Rich Paints”, Steinmetz J. R., Modern Paint and Coatings, June 1983. If alkaline materials are introduced in the liquid part of an acidic-stabilised silicate binder, the liquid becomes neutral and the silanol groups become reactive and an instable liquid is formed.
It is well known to the person skilled in the art that depending on the type of acidic-stabilised alkyl silicate binder and alkaline material introduced into the silicate binder, the stability in the liquid part could vary from no more than 6 months stability to almost immediate gelling of the product.
Examples of alkaline materials are fillers such as feldspar, talc, mica, dolomite, calcite, bauxite, or various types of silicate materials.
In practise, the person skilled in the art will refrain from introducing alkaline materials in to the composition in order to secure storage stability. The person skilled in the art will select ingredients among materials giving a pH in the neutral area or a pH below 7 when suspended or dissolved in water.
Acidic stabilised alkyl silicate coatings are easy to apply with standard spraying equipment. However, spraying a silicate paint composition is a little different from conventional paints. Normally, paints have a tendency to build up in corners on welding seams and in difficult accessible areas, a problem which is even more pronounced if the silicate paint composition contains a metallic pigment e.g. zinc. These products are formulated with PVC above the CPVC ratio to ensure sufficient contact between the zinc particles for galvanic protection. Silicate coatings with metallic pigment will therefore have a higher risk for mud cracking if they are applied in too high film thickness. Consequently, extra efforts and man hours are often spent here in order not to end up with too high dry film thicknesses.
Existing silicate products are specified in a total dry film thickness of 40 to 125 μm, typically 75 μm, but it is unavoidable to get overlaps and high dry film thickness in corners and edges. Especially in association with the interior of the angles formed between the structural elements there is a risk of getting far higher dry film thicknesses than specified. Often it is necessary to go after with brushes on the critical areas in order to avoid mud-cracking. Removing excess paint manually by brush is time consuming and not the ideal solution but the alternative is worse: Too high coating thickness, often exceeding normal acceptance limits, causes high curing shrinkage stress and may cause mud-cracking.
When the silicate paint composition is applied in too high film thickness, mud cracking appears. This happens when the narrowly defined correlation between the applied film thickness, the drying process, and the curing is “out of balance”.
In the past asbestos fibres and fibrous calcium silicate have been used in zinc alkyl silicate compositions (U.S. Pat. No. 3,056,684).
In recent years faster curing products have appeared comprising accelerators such as zinc chloride or magnesium chloride. Fast curing lowers the overall processing time which has become a very important parameter for paint applicators and contractors. However it has only been possible to increase curing up to a certain level as the fast curing has a negative impact on the cracking level (internal stress). The amount of zinc chloride that can be added is therefore limited.
The curing process starts on the surface of the coating. When curing is too fast the soft uncured part of the coating will not have sufficient strength to “carry” the stress built up during curing and mud cracks will develop on the surface leading to subsequent loss of adhesion, cohesion and corrosion problems
There is a need for a storage stable, acid catalysed alkyl silicate with increased flexibility, enabling faster processing without mud cracking.
It has now surprisingly been discovered that it is possible to incorporate substantial amounts of alkaline fibres into acidic-stabilised alkyl silicate composition without any stability problems.